What is Copyleft?

The simplest way to make a program free software is to put it in the
public
domain, uncopyrighted. This allows people to
share the program and their improvements, if they are so minded. But
it also allows uncooperative people to convert the program into
proprietary
software. They can make changes, many or few,
and distribute the result as a proprietary product. People who
receive the program in that modified form do not have the freedom that
the original author gave them; the middleman has stripped it away.

In the GNU project, our aim is
to give all users the freedom to redistribute and change GNU
software. If middlemen could strip off the freedom, our code might “have
many users,” but it would not give them freedom. So instead of
putting GNU software in the public domain, we “copyleft”
it. Copyleft says that anyone who redistributes the software, with or
without changes, must pass along the freedom to further copy and
change it. Copyleft guarantees that every user has freedom.

Copyleft also provides an
incentive
for other programmers to add to free software.
Important free programs such as the GNU C++ compiler exist
only because of this.

Copyleft also helps programmers who want to contribute
improvements to
free software get permission to
do so. These programmers often work for companies or universities
that would do almost anything to get more money. A programmer may
want to contribute her changes to the community, but her employer may
want to turn the changes into a proprietary software product.

When we explain to the employer that it is illegal to distribute the
improved version except as free software, the employer usually decides
to release it as free software rather than throw it away.

To copyleft a program, we first state that it is copyrighted; then we
add distribution terms, which are a legal instrument that gives
everyone the rights to use, modify, and redistribute the program's
code, or any program derived from it, but only if the
distribution terms are unchanged. Thus, the code and the freedoms
become legally inseparable.

Proprietary software developers use copyright to take away the users'
freedom; we use copyright to guarantee their freedom. That's why we
reverse the name, changing “copyright” into
“copyleft.”

Copyleft is a way of using the copyright on the program. It
doesn't mean abandoning the copyright; in fact, doing so would make
copyleft impossible. The “left” in
“copyleft” is not a reference to the verb “to
leave”—only to the direction which is the mirror image of
“right”.

An alternate form of copyleft, the GNU
Affero General Public License (AGPL) is designed for
programs that are likely to be used on servers. It ensures that
modified versions used to implement services available to the public
are released as source code to the public.

The GNU Free Documentation License (FDL)
is a form of copyleft intended
for use on a manual, textbook or other document to assure everyone the
effective freedom to copy and redistribute it, with or without
modifications, either commercially or noncommercially.

The appropriate license is included in many manuals and in each GNU
source code distribution.

All these licenses are designed so that you can easily apply them to
your own works, assuming you are the copyright holder. You don't have
to modify the license to do this, just include a copy of the license
in the work, and add notices in the source files that refer properly
to the license.

Using the same distribution terms for many different programs makes it
easy to copy code between various different programs. When they all
have the same distribution terms, there is no problem. The Lesser
GPL, version 2, includes a provision that lets you alter the
distribution terms to the ordinary GPL, so that you can copy code into
another program covered by the GPL. Version 3 of the Lesser GPL is
built as an exception added to GPL version 3, making the compatibility
automatic.

If you would like to copyleft your program with the GNU GPL or the GNU
LGPL, please see the license
instructions page for advice. Please note that you must use the entire
text of the license you choose. Each is an integral whole, and
partial copies are not permitted.

If you would like to copyleft your manual with the GNU FDL, please
see the instructions at the
end of the FDL text, and
the GFDL instructions page. Again,
partial copies are not permitted.

It is a legal mistake to use a backwards C in a circle instead of a
copyright symbol. Copyleft is based legally on copyright, so the work
should have a copyright notice. A copyright notice requires either
the copyright symbol (a C in a circle) or the word
“Copyright”.